1. Field of the Invention
The invention relates to a process and a device for spinning cellulose carbamate solutions by extruding the solution through a spinneret into a precipitation bath, coagulating the cellulose carbamate fibers thus formed by bringing them in contact with the precipitation medium, and drawing off the fibers by mechanical means after discharge from the precipitation bath.
2. Summary of the Related Art
It is known that aqueous alkaline solutions of cellulose carbamate can be spun in acidic, alkaline, or alcoholic precipitation baths in accordance with the techniques used with viscose fibers (European Patents Nos. 97,685 and 103,618; British Patents Nos. 2,164,941, 2,164,942 and 2,164,943; O. T. Turunen, Lenzinger Berichte [August 1985] no. 59, pages 111-115). However, these publications are concerned with the composition of the precipitation baths and do not give details on the spinning apparatus.
Various techniques are known for spinning viscose fibers, including tube spinning where the fibers are guided through a horizontal cylindrical tube within the precipitation bath after leaving the spinneret (Z. A. Rogowin, Chemiefasern [Manmade Fibers], Thieme-Verlag [1982], page 134), and funnel spinning, where the fibers, after leaving the spinneret, are sent together with the precipitation medium through a vertical funnel that is conical over the entire length (K. Gotze, Chemiefasern nach dem Viskoseverfahren [Manmade Fibers by the Viscose Method], Springer-Verlag, 3.sup.rd edition, volume 2, pages 602-607). Similarly designed spinning funnels have also been described for dry-wet spinning of cellulose solutions in aqueous N-methylmorpholine N-oxide (NMMO) (German Patents Nos 4,219,658 C and 19,515,136 A).
An important disadvantage of the tube spinning method is the lack of exchange of the precipitation medium between the inside and outside of the tube and the resulting accumulation within the tube of solvent and possible cleavage products from the spinning solution. The German Patent No. 39 04 541 A1 therefore proposes to circulate the precipitation medium through a spinning tube, preferably from bottom to top for a vertical tube, wherein the velocity of flow of the precipitation medium at the exit of the spinning tube is set to a value lower than the draw-off speed of the fibers, and corresponding to a velocity to speed ratio of 0.15 to 0.95. This helps the exchange of the precipitation medium within the tube, but causes on the fibers a force opposed to the draw-off direction, which influences the morphology of the fiber.
In the spinning funnel method, however, the flow velocity of the precipitation medium within the funnel is so high as to cause the fibers to stretch due to the fiber/fluid friction. In both cases, the freshly extruded fibers are exposed to turbulence occurring mainly at the nozzle surface immediately after leaving the spinneret and before entering the spinning tube or the spinning funnel, which in turn has a negative effect on fiber uniformity.
Whereas simultaneous coagulation and saponification of the cellulose xanthogenate to regenerated cellulose fibers with a much greater strength and coherence occurs on entering the precipitation bath, cellulose carbamate undergoes only coagulation. The freshly spun cellulose carbamate fibers are consequently much more susceptible to turbulence or other disturbances and stretching forces within the precipitation bath than are regenerated cellulose fibers. Therefore, direct transfer of the spinning techniques used with viscose or cellulose-NMMO solutions to spinning solutions of cellulose carbamate is either impossible or leads to poor quality fibers.
In view of the foregoing, therefore, improved techniques applicable to spinning solutions of cellulose carbamate are desired.